1. Field of the Invention
The present invention concerns a liquid crystal device for use in switching devices, display devices, etc. and, more specifically, it relates to a liquid crystal device suitable to a display device of large area having a matrix picture element structure.
2. Description of the Prior Art
A ferroelectric liquid crystal showing the chiral smectic C phase has such a nature that helices are released and optical axes are arranged in one direction when sealed in a gap between two sheets of glass substrates having transparent electrodes formed at the inside. In a liquid crystal cell comprising such a ferroelectric liquid crystal, since the angle of the optical axis of liquid crystal molecules varies depending on the electric field applied thereto, black and white display at high contrast is possible by operating the cell put between two sheets of polarization plates.
The display system of utilizing such birefringence of the ferroelectric liquid crystal is evaluated as being suitable to the matrix display of large picture area since the two conditions, i.e., a display state and a non-display state are switched at high speed depending on the polarity of impressed voltage and, in addition, each of the displayed states is storeable.
However, it has been difficult for the liquid crystal cell of large area to obtain uniform orientation to the entire liquid crystal molecules and a defect such as interlayer staggering of the smectic C phase is liable to occur. In addition, in the case of the smectic C phase, since the viscosity of the liquid crystal is high and the degree of order for the molecular arrangement is also high, different from those in the nematic phase, it has been difficult for the defects once formed to recover spontaneously into the initial state of orientation.
Generation of such a defect gives undesired effects on the display characteristics such as reduction in the display contrast and variation of the threshold voltage.
Further, in order to utilize the expected storeability of the display in the liquid crystal device utilizing the birefringence of the ferroelectric liquid crystal, it is necessary that ON-OFF states can freely be rewritten. However, since the threshold voltage for writing the ON state often differs from that for writing the OFF state because of the deviation in the oriented state of molecules, etc. and since physical properties such as spontaneous polarization, viscosity and elastic constant of the ferroelectric liquid crystal are greatly dependent on the temperature, the temperature range in which the storeability can be effectively utilized is actually narrow.
FIG. 7 shows a relationship between the spontaneous polarization and the temperature and FIG. 8 shows a relationship between the bistable threshold voltage and the temperature, which have been examined for CS-1017 liquid crystal which is a ferroelectric liquid crystals (manufactured by Chisso Co., Ltd.). It can be seen that the temperature dependency is remarkable in either of the cases.
As one of the methods for resolving such problems, TS mode for causing the ferroelectric liquid crystal to reverse the spontaneous polarization by applying AC voltage thereby scattering light has been is proposed (refer to Japanese Patent Laid-Open Sho 60-195521).
In the case of the TS mode, however, although it has a merit that the orientating treatment is not required and the cell can be manufactured with ease, there has been a problem that a driving voltage as high as 50-100 V is necessary and, in addition, time divisional driving is impossible since there is no threshold characteristic between the transmission state and the scattering state.
In view of the above, there has also been proposed a method of causing light scattering by inducing a helical texture (refer to Japanese Patent Laid-Open No. Sho 61-267738). In this method, however, since there is no storeability of display in the light transmission state, there is a problem that a DC electric field has to be applied continuously for maintaining the light transmission state, which degrades the cell in a short period of time.